1. Field of the Invention
The present invention relates to a continuous casting mold for manufacturing slabs, particularly for manufacturing thin slabs. The continuous casting mold includes water-cooled short side walls and long side walls, wherein the long side walls are composed of copper plates, and wherein a casting funnel is formed between the long side walls in an area where the long side walls extend parallel to each other and extend outwardly toward the two short side walls and cooling grooves are provided on the other outer or rearward sides of the long side walls.
2. Description of the Related Art
The thin slabs manufactured in continuous casting plants serve as preliminary strips for producing hot wide strip with final thicknesses of between, for example, 1-12 mm. This is carried out in a heat on conventional, so-called CSP plants or compact strip production plants in which the casting machine is combined with an equalizing furnace and a subsequent rolling train in a line (see, for example, xe2x80x9cStahl und Eisenxe2x80x9d 108 (1988) No. 3, pages 99 to 109). In this process, the funnel-shaped mold together with the pouring pipe immersed in the casting funnel constitute the core element of the CSP casting process. The heat removal in the area of the molten metal level and the service life of the mold are of particular importance because the surface quality of the thin slab strand is to a significant extent influenced by the first solidification starting in the mold of the liquid steel supplied through the pouring pipe. These criteria are applicable to the same extent to the manufacture of slabs having the range of dimensions of 320 to 150xc3x973,000 to 800 mm by means of a plate-type mold which then usually does not require a pouring funnel.
The strand shell growth is determined to a significant extent by the casting powder, the copper wall thickness between the cooling duct and the work side as well as the flow velocity in the cooling ducts. It has been found that an important reason that longitudinal cracks occur in the strand shell is a non-uniform and/or excessive heat removal over the width and height in the upper mold portion. The uneven heat removal is due to different thicknesses of the lubricating film between the strand shell and the copper wall.
For reasons of quality, copper walls having a thickness of, for example, 25 mm and 15 mm are used in molds of CSP plants; the respective dimension depends essentially on the strand dimensions and the casting speed which determine the heat removal required for the formation of the strand shell.
Therefore, it is the primary object of the present invention to provide a continuous casting mold of the above-described type which makes it possible to improve the surface quality of the thin slab to be cast and in which the occurrence of longitudinal cracks can be avoided or at least minimized.
In accordance with the present invention, the rearward sides of the long side walls are each reinforced by a wall armor starting at a mold location below the lowest molten metal level, wherein the thickness of the wall armor decreases in the strand casting direction from a greatest thickness at the mold location below the lowest molten metal level.
In continuous casting, the molten steel which is introduced into the mold and solidifies at the mold walls leads to the known temperature profile, i.e., the so-called temperature bulge in the mold plate. In contrast to the otherwise very rapid vertical temperature drop following this temperature bulge in an area up to about 100 mm below the molten metal level, the wall armor according to the present invention makes it possible to reduce this very rapid temperature drop below the temperature bulge, wherein the starting point of the armor is determined by taking into consideration the contour of the meniscus at the molten metal level which depends on the casting speed, and wherein the armor begins with its greatest thickness, following a previous increase or transition over a short distance, approximately 20 to 50 mm below the meniscus in the casting direction.
By increasing the copper wall thickness below the meniscus, the heat removal at this location is reduced and the generation of longitudinal cracks is counteracted by a smaller thermal load on the strand shell. This is because the armor or increased wall thickness which already starts in the area of the temperature bulge prevents an excessively high heat removal, wherein the heat removal is only once again steadily increased with the conically downwardly decreasing thickness of the long side wall, so that an equalization can be achieved in the sense that the temperature difference is such that there is no danger of longitudinal cracks.
In accordance with an advantageous further development of the invention, an upper end portion of the wall armor of limited length extends parallel to the mold wall. This further advantageously influences the equalization of the temperature drop. This is because this upper end portion of the wall armor with maximum thickness, which depending on the requirements may be 3 to 15 mm, and which after a short increase has about a length of 20 to 100 mm, is located at a location where the heat removal otherwise is too high, and, thus, the armor with increased thickness prevents an excessive heat removal from the cooling water side.
In accordance with a feature of the invention, the wall armor decreases from the transition from the pouring funnel to the mold end portion which follows the pouring funnel in the strand casting direction and which determines the shape of the strand. In this manner, it is achieved that the copper wall thickness in the front of water decreases over the length of the cooling grooves and, thus, the heat removal can be influenced in a targeted manner over the height of the mold.
In accordance with a further development of the invention, the thickness of the wall armor varies over the width and height of the mold in dependence on the temperature distribution in the mold. The resulting varying pattern of the groove geometry in the copper plate advantageously influences the desired temperature equalization of the height and width of the mold provided by the armor configured in accordance with the temperature decrease in dependence on the casting parameters.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.